1. Field of the Invention
The present invention pertains to a process for preparing printed circuits, and in particular a process for preparing a laminated printed circuit.
2. Description of the Prior Art
Printed circuits and their modifications, ranging from simple printed and etched plates to flexible circuit boards to multilevel circuit boards, are an integral part of modern technology. They consist, in principle, of a base plate or carrier plate made of an electrically insulating material on which conductor strips made of an electrically conducting, metallic material are applied according to a desired lay-out. The conductor strips can be located in different planes, and they are connected with each other by so-called pass through or through-hole plates. Various processes are available for preparing the conductor strips; the most important of these are the additive process, the subtractive process, and the semiadditive process. It is a common feature of all processes that the conductor strips ultimately lie raised on the substrate. In the case of the application of printed circuits, e.g., for switches or potentiometers, this is bothersome, because the slide contact has to steadily clear differences in height, which results in more rapid wear. There has therefore been an increased tendency to embed the conductor strips, resistor strips and the like into the substrate by the so-called reverse lamination technique.
The principle of the reverse lamination technique is already known from DE-PS [West German Patent] No. 738,414. It was observed during the preparation of resistor layers that the conductor particles in the still liquid layer become deposited preferably in the substrate under the effect of gravity before hardening, while the surface has a reduced concentration of conductor particles. The resistivity and also the current distribution are consequently not uniform over the entire cross section. This affects the noise characteristics of the resistor layer if it is used in a potentiometer. Attempts have now been made to find a resistor layer in which the highest density of the conductor particles occurs immediately on the surface, and in which the density decreases in the direction of depth. It has been proposed that this be achieved by applying the resistor mass in liquid state first on a temporary substrate, the intermediate carrier. Under the effect of gravity the conductor particles sink down to the side of the substrate. After the hardening of the resistor mass the layer is lifted off from the intermediate carrier, and atttached on the final carrier with the side which has a lower content of conductor particles. A body with a surface polished to a high gloss, e.g., glass, is used as the intermediate carrier. The surface properties of the layer resistor are also dependent on the roughness of this surface. This known process is also recommended when several resistor layers are to be applied one on top of the other. Mechanical operations are eliminated in this process. If the layer thickness is great enough and the duration of settling is long enough, the density of the conductor particles decreases so strongly that this side acts as an insulator. It is now possible to take a metal plate which has a favorable heat conduction as the final substrate.
A reverse lamination process is described in DE-OS [West German Offenlegungsschrift] No. 30 31 751, in which a metal foil consisting of an etchable material, e.g., aluminum or copper, is used as the intermediate carrier. The intermediate carrier can also be a plastic film or it can consist of anodized aluminum or of glass. That side of the intermediate carrier on which the resistor mass is to be applied must be polished to a high gloss in this case as well. After the resistor mass is applied by a screen printing technique in the desired lay-out, hardening is carried out. The conductive layer is transferred onto the final substrate directly or by means of an intermediate adhesive layer. After transfer the intermediate carrier is removed by etching or dissolution of the intermediate carrier in water, or by lifting off in the case of glass. In order to guarantee a continuous movement of the slide contact, the space between the resistor layer and the corresponding conductor strips is filled out with a binder after hardening of the resistor layer.
Another process for preparing copper-lined laminates by the use of the reverse lamination technique is known from DE-OS No. 31 31 688. An aluminum carrier plate is coated with a substance acting as a parting agent, which has a tendency to form a relatively weak bond with copper. Silicon dioxide, silicon oxide or soda-lime window glass are used as substances. The coating is carried out by spraying, by chemical vapor deposition, or by electron beam sputtering techniques. After coating of the aluminum carrier plate with this substance, a copper coating is applied by spraying or another vapor deposition technique. The copper layer is subsequently subjected to an electrolytic treatment to form a roughened surface. This surface is then flash-coded with zinc galvanically. The lamination step is carried out by pressing of the carrier plate thus treated with a glass-epoxy-preimpregnated web material. The carrier plate is subsequently removed by stripping off mechanically. The parting agent remains on the carrier plate, so that the metal surface of the laminate becomes exposed.
Another process for preparing printed circuits which contain integrated electrical resistors, contacts and corresponding conductor strips, is described in DE-OS No. 31 35 554. The reverse lamination technique is used in this case as well, and the resistors, contacts and the conductor strips are applied on an intermediate carrier in the desired lay-out. This is followed by hardening. The object is then laminated onto a substrate with the coated side by using an epoxy resin adhesive. The intermediate carrier is finally removed by etching in a final process step.